Power adapter EMI rectification
| The power adapter EMI is indeed difficult to understand and it is difficult to have an accurate paper design. However, through research, we can still know the general trend to guide the design. Generally speaking, the power adapters put on the market meet the corresponding EMI standards. Of course, this refers to It is a civil standard. If the military standard test is used, the corresponding will be stricter, so the EUT test is normal. Several factors that mainly affect the EMI and EMC of the power adapter are the switching circuit, circuit board design and grounding circuit of the power adapter, and various functional circuits such as the switching circuit. |
When encountering the problem of conduction test, the first step is to locate the noise component mainly in differential mode or common mode. The usual test equipment can be used to distinguish the difference common mode component, but I find it too troublesome and the test results Relative values are not necessarily instructive. The easiest way is to connect an X capacitor in parallel with the input port, tens of nF to hundreds of nF. If the test of the frequency band of interest passes, it means that the noise interference is mainly differential mode interference, or more accurately, by reducing the voltage The differential mode component can certainly solve the problem.
As for how to reduce the differential mode component, there are only two solutions. One is to strengthen the differential mode filtering, and the other is to reduce the differential mode noise at the source. The following analyzes the solution for a typical design.
The figure above is part of the schematic diagram of the typical application of the low-power PFC flyback-free power adapter. The device shown in the third box is the source of differential mode noise: 1) the current flowing through the transformer 2) the RCD absorption circuit For general design, leakage inductance is usually controlled relatively small, the former is the main contributor to differential mode current.
The first box and the second box both play the role of differential mode filtering. The first box uses the differential mode components of the X capacitor and the common mode inductor for differential mode filtering, and the second box uses the differential mode inductor and two An energy storage electrolytic capacitor is π-filtered. In typical applications, the two types generally do not appear at the same time, that is, the typical applications usually have the following two types:
In the first application, after a part of the differential mode current is absorbed by the energy storage capacitor, the remaining components are all filtered by the common mode inductance Lcm and the X capacitor Cx. This design is usually applied to the occasion that requires conducting EMI grounding test. Large, the corresponding differential mode component is also relatively large, there are three measures to reduce the differential mode component from the filtering angle: 1) reduce the ESR and ESL of the storage capacitor C1; 2) increase the X capacitor Cx capacity; 3) increase the differential mode inductance Lcm differential mode component.
The first measure is limited by the characteristics of the electrolytic capacitor itself and has little room to play. The second measure is limited by the size of the space and the standby power consumption (larger X capacitors require smaller discharge resistance). Big. We can increase the differential mode component by simultaneously increasing the common mode inductance (using a magnetic ring with a higher permeability, using a larger size magnetic ring, and increasing the number of winding turns), or by appropriately reducing the common mode inductance To increase the differential mode inductance, the simple method is: for a ring-shaped common mode inductor, a silicon steel sheet can be inserted between the two windings to provide a path for the magnetic ring to generate differential mode magnetic flux. This solution does not It will increase the size and hardly increase the cost. The disadvantage is that it will sacrifice a certain amount of common-mode inductance, but in general applications, the margin of the common-mode inductance is sufficient.
In the second application, the differential mode current is completely dependent on the energy storage capacitor and the differential mode inductor. Generally speaking, this design has a strong filtering capacity for the differential mode component and the cost is relatively low, because the differential mode inductor can be used cheaply. I-shaped inductor. There are two main problems with this design: 1) The characteristics of the ESR of the electrolytic capacitor will cause poor differential mode filtering at low and normal temperatures, and the margin will become sufficient after high temperature or aging for a period of time; 2) This design increases The surge current pressure of C1 also increases the ripple current pressure of C2. Since the electrolytic capacitor capacity used for energy storage is allocated to both ends of the differential mode inductor, most surge currents are absorbed by C1 during the surge test, resulting in an increase in the probability of C1 failure; on the other hand, most high-frequency The component ripple currents are all absorbed by C2, which causes the temperature rise of C2 to be significantly higher than C1 under normal operation, and the life of C2 is affected.
Despite the above problems, the method of using the energy storage capacitor to form a π-type filter is widely used in low-power power adapter products due to the good effect of differential mode filtering, no X capacitor, and low cost. Another method to solve the above problem is to replace C2 with a low-capacity film capacitor with strong ripple current capability. In this way, without reducing the differential mode filtering capability, a large-capacity C1 can be used to enhance the surge current tolerance. ability.
If it is a CLC behind a PFC bridge, it is used for several hundred W. A CLC without PFC can be used for several tens of W. It is just that high power generally requires the output to be grounded and the input to have a large common mode. Common mode differential mode components to filter differential mode noise.
The above measures are all to reduce the differential mode component from the perspective of filtering. In addition, it is also possible to think of a solution from the source of the noise.
For the flyback power adapter, the differential mode component mainly comes from the ripple current of the switching frequency. For the CCM scheme, the amount of inductance can be appropriately increased or the frequency can be increased to increase the depth of the CCM to reduce the ripple current.
In addition, from a testing perspective, we can avoid the first fundamental wave / second harmonic / third harmonic of the conducted test frequency band by reducing the frequency. Commonly used power supply control ICs usually take this into consideration. The frequency is 65KHz (that is, the second harmonic after taking into account the error will not enter the conduction test band) or 130KHZ (that is, the switching frequency after taking into account the error will not enter the conduction test band).
If the 5V2A power adapter does not need to output ground to measure emi, the common mode is definitely not needed.
Now let's talk about the problem of conducted common mode noise.
A personal summary of conducted common-mode noise typically has several paths:
1) dv / dt with high switching point is directly coupled to the input L / N line
2) The dv / dt with a high switch moving point is coupled from the primary side to the ground plane
3) dv / dt with high switching point is coupled to the secondary side through the transformer, and then coupled from the secondary side output to the ground plane
4) The high di / dt of the switching circuit generates a magnetic field, which is coupled to the input L / N line
The above ground plane refers to the earth. The first three types are electric field coupling, the transmission path is parasitic capacitance; the fourth type is magnetic field coupling.
We can summarize the main control technologies for the power adapter EMC / EMI: grounding methods, circuit measures, EMI filtering, component selection, shielding, and PCB board anti-interference design.If these issues can be properly and reasonably addressed, To solve, that is to solve the problem of electromagnetic interference and electromagnetic compatibility of the power adapter at the source.
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|| Release time: 2020.01.02 Source:|
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